To get chocolate fondant I add about a 1/4 a cup of cocoa powder (I like to use Hershey's dutch process cocoa for a dark rich brown) to the soupy marshmallow mixture. Then mix it with your powdered sugar like regular mmf except you won't need as much powdered sugar since the cocoa dries it out a little more. Just don't add melted chocolate because it does something to the the mixture and makes it crumbly.